The present embodiments relate to wireless communications systems and are more particularly directed to synchronizing a receiver to a transmitter.
Wireless communications have become prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” More particularly, CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (“WCDMA”). WCDMA includes alternative methods of data transfer, one being time division duplex (“TDD”) and another being frequency division duplex (“FDD”). The present embodiments may be incorporated in either TDD or FDD and, thus, both are further introduced here. TDD data are transmitted in one of various different forms, such as quadrature phase shift keyed (“QPSK”) symbols or other higher-ordered modulation schemes such as quadrature amplitude modulation (“QAM”) or 8 phase shift keying (“PSK”). In any event, the symbols are transmitted in data packets of a predetermined duration or time slot. Within a TDD data frame having 15 of these slots, bi-directional communications are permitted, that is, one or more of the slots may correspond to communications from a base station to a user station while other slots in the same frame may correspond to communications from a user station to a base station. Further, the spreading factor used for TDD is relatively small, whereas FDD may use either a large or small spreading factor. FDD data are comparable in many respects to TDD including the use of 15-slot frames, although FDD permits a different frequency band for uplink communications (i.e., user to base station) versus downlink communications (i.e., base to user station), whereas TDD uses a single frequency in both directions.
By way of illustration, a prior art FDD frame FR is shown in FIG. 1. Frame FR is a fixed duration, such as 10 milliseconds long, and it is divided into equal duration slots. In the past it was proposed in connection with the 3G standard that the number of these equal duration slots equals 16, while more recently the standard has been modified such that each frame includes 15 equal duration slots. Each of the 15 slots has a duration of approximately 667 microseconds (i.e., 10/15 milliseconds). For the sake of reference, 15 such slots are shown in FIG. 1 as SL1 through SL15, and slot SL1 is expanded by way of example to illustrate additional details.
To accomplish the communication from a user station to a base station, the user station must synchronize itself to a base station. This synchronization process is sometimes referred to as acquisition of the synchronization channel and is often performed in various stages. The synchronization channel, shown in expanded form as SCH in FIG. 1, includes two codes, namely, a primary synchronization code (“PSC”) and a secondary synchronization code (“SSC”), as transmitted from a base station. As shown in frame FR of FIG. 1, both the PSC and SSC are included and transmitted in slot SL1 for frame FR, while it should be further understood for FDD communications that the SCH is also included in each of the remaining slots SL2 through SL15, although those slots are not shown in expanded form so as to simplify the Figure. The PSC is presently a 256 chip Golay code and the same PSC code is transmitted from numerous base stations. Each base station group transmits a unique set of SSC code words. Within each slot such as slot SL1, the PSC and SSC may be offset by some period of time, Toffset, within the slot. Under the present standard, Toffset is the same for both the PSC and the SSC. However, in alternative implementations, the PSC and SSC may be offset from one another, in which case it may be stated that the PSC has an offset Toffset1 from the slot boundary and the SSC has an offset Toffset2 from the slot boundary. For the sake of an example in the remainder of this document, assume that Toffset1=Toffset2.
The synchronization process typically occurs when a user station is initially turned on and also thereafter when the user station, if mobile, moves from one cell to another, where this movement and the accompanying signal transitions are referred to in the art as handoff. Synchronization is required because the user station does not previously have a set timing with respect to the base station and, thus, while slots are transmitted with respect to frame boundaries by the base station, those same slots arrive at the user station while the user station is initially uninformed of the slot and frame boundaries among those slots. Consequently, the user station typically examines either one slot or one frame-width of information (i.e., 15 slots), and from that information the user station attempts to determine the location of the actual beginning of the frame (“BOF”), as transmitted, where that BOF will be included somewhere within the examined frame-width of information. Further in this regard, the PSC is detected in a first acquisition stage, which thereby informs the user station of the periodic timing of the communications, and which may further assist to identify the BOF. The SSC is detected in a later acquisition stage, which thereby informs the user station of the data location within the frame. The actual base station is identified from the third stage of the synchronization process, which may involve correlating with the midamble (in TDD) or long code (in FDD) from the base station transmissions depending on the type of communication involved. Once the specific long code/midamble from that group is ascertained, it is then usable by the user station to demodulate data received in frames from the base station.
Returning now to frame FR in general, a further discussion is presented concerning the prior art approach of detecting the PSC in a first acquisition stage. Specifically, in order to locate the PSC in a prior art FDD frame, a user station typically samples one slot-width of information and performs a PSC correlation on the sampled slot and the PSC is determined to be located within the sampled information at the position identified as having the largest correlation. For example, this technique may be implemented by applying the received information to a matched filter having the 256 chip PSC as coefficients to the filter, and then observing the absolute value (i.e., the energy) of the output of the filter. To further refine this approach, often an average is taken for successive slot-widths of correlated measurements. In this approach, the average peak over time of those correlations correspond to the location of the synchronization channel within the collected information.
While the above-described approach to stage 1 acquisition of the PSC has provided satisfactory results, the present inventors have observed various drawbacks related to that approach. Specifically, the number of correlations measured is usually twice the total chip rate, that is, the PSC correlation is measured twice for each chip included within the frame width of information. Further, the results of the PSC correlations are typically stored within a buffer as those correlations are measured. For example, for a chip rate of 3.84 Mcps, then the PSC correlations are at a rate of 7.68 million correlations per second. Further, if a slot has a duration of approximately 667 microseconds (i.e., 10 milliconds/15 slots), then a total of 5,120 samples (i.e., 2×3.84×666.666666667=5,120) are taken per slot. Also, recall it is noted above that often an average is taken for successive slots; thus, to implement this approach in the prior art, a buffer is used for a set of samples, with the average then taken by accumulating values into that buffer. In this approach, therefore, the buffer must accommodate the total number of samples taken and, thus, for the numeric example provided, a buffer having a total of 5,120 elements must be provided to store the PSC correlation values. The requirement of a large buffer may provide various disadvantages, such as increased complexity and cost. Additionally, since the user station is typically a portable and relatively small device, then resource allocation may be even more complex and, thus, disadvantages such as those just mentioned are even more pronounced in the portable device.
In view of the above, there arises a need to provide an approach for correlation measurements in a wireless system with reduced resource requirements, as is achieved by the preferred embodiments discussed below.